Many working machines are hydraulically driven. Examples of hydraulically driven working machines include hydraulic shovels. Generally, in hydraulic shovels, a shovel is driven, an upper-part turning body is caused to turn, and a lower-part traveling body is caused to travel using hydraulic actuators (hydraulic cylinders and hydraulic motors). In general, a hydraulic pressure supplied to hydraulic actuators is often generated by a hydraulic pump whose drive source is an engine. In this case, the outputs of the hydraulic actuators are determined by the output of the engine.
A full capacity of the engine is not always required and, for example, 90% or 80% of the capacity is often sufficient for the operations of hydraulic shovels. Therefore, the operation mode of the hydraulic shovel is changed according to work loads so as to optimize engine output control for each of different work loads, thereby efficiently driving the engine to improve fuel consumption.
For example, it is made possible to set different operation modes such as “a high load mode” for performing a load operation corresponding to the maximum output of the engine, “a normal load mode” for performing a normal-load operation, and “a low load mode” for performing a light-load operation. Then, iso-horsepower control is performed so as to equalize the drive torque required by the hydraulic pump to drive the hydraulic actuators with the output torque of the engine in each operation mode, thereby efficiently utilizing the output of the engine to improve fuel consumption.
In general, hydraulic shovels are equipped with an engine whose maximum output is equal to its output in the “high load mode.” However, the operation is far less often performed in the “high load mode” than in the “normal load mode.” Therefore, when the hydraulic shovel is operated in the “normal load mode,” the engine output is less than its maximum. In other words, the hydraulic shovel is equipped with a large engine whose output is excessive for the operation in the “normal load mode.”
In recent years, there has been a demand for reduction in the amount of fuel consumed by the engine in hydraulically driven working machines including the above-described hydraulic shovel. Simple reduction in the size of the engine would result in insufficient hydraulic output at the time of operating in the “high load mode.” Therefore, so-called hybrid hydraulic construction machines have been developed that are equipped with an engine, a generator driven by the engine, an electric power accumulating part charged by the generator, and an electric motor driven with the electric power of the electric power accumulating part. (See, for example, Patent Document 1.)